Fuel cells are units for directly converting chemical energy stored in hydrocarbon fuel into electric energy through an electrochemical reaction. That is, fuel cells are units for directly converting chemical energy into electric energy through a hydrogen oxidation reaction in an anode and an oxidation reduction reaction in a cathode. For these reactions, a fuel gas (hydrogen) has to be supplied into an anode of a fuel cell stack, and air (oxygen) has to be supplied into a cathode of the fuel cell stack. A fuel cell system for producing electricity through above-described reactions may include a fuel cell stack, a mechanical balance of plant (MBOP), and an electrical balance of plant (EBOP). The fuel cell stack may be a unit for producing electricity through the electrochemical reaction, the MBOP may be a unit for supplying oxygen and hydrogen into the fuel cell stack, and the EBOP may be a unit for converting a DC power applied into the fuel cell stack into an AC power through an inverter to supply the converted AC power to desired units.
High-temperature fuel cells such as molten carbonate fuel cells (MCFCs) may produce electricity through following processes. Referring to FIG. 3, a fuel gas such as a natural gas generally contact sulfur (S). The sulfur (S) may have a bad influence on a reforming catalyst of a reformer 20 that will be described later. Thus, the fuel gas has to be desulfurized first through a desulfurizer (not shown). Also, a reforming reaction occurring in the reformer 20 requires water. Thus, since liquid water may damage the reforming catalyst, gaseous water together with the fuel gas has to be supplied into the reformer 20. Thus, the fuel gas may be humidified while passing through a humidifying heat exchanger. The humidifying heat exchanger 10 may evaporate liquid water by using a high-temperature cathode exhaust gas that is exhausted from a cathode 44 of a fuel call stack. As a result, the desulfurized and humidified fuel gas may be reformed into hydrogen in a pre-reformer 20. The hydrogen may be heated to a required temperature through heat-exchange in a heat exchanger 30. Then, the hydrogen is supplied into an anode 42 of the fuel cell stack.
Also, as described above, air (oxygen) has to be supplied into the cathode 44 of the fuel cell stack. For this, the fuel cell may generally supply an oxidizer supply gas from an air source such as the atmosphere into the cathode 44. Here, the oxidizer supply gas may be air. In some cases, the oxidizer supply gas may be a gas in which a portion of a cathode exhaust gas is recycled and then mixed with air. However, the oxidizer supply gas is needed to be heated to an adequate temperature so as to be supplied into the cathode 44. To heat the oxidizer supply gas, a catalytic combustor 50 for burning a non-reaction fuel gas of an anode exhaust gas to heat the oxidizer supply gas may be used. The heated oxidizer supply gas may be supplied into the cathode 44 of the fuel cell stack.
However, MCFCs that produce electricity through the above-described processes may generate heat in addition to the electricity. Thus, in a fuel cell system using high-temperature fuel cells such as the MCFCs, efficiency of the entire system may vary according to a use method of the heat. As a result, methods for utilizing the heat are being variously taken to improve the efficiency of the entire system. For example, a method in which the heat is used as heating source is being attempted. However, for utilizing the heat as the heating source, the fuel cell system has to be installed in a place that is not far away from a place at which the heating source is needed.
To overcome the foregoing limitation, methods for utilizing the heat in a heat engine (for example, a gas turbine) or an organic rankine cycle (ORC) device are being taken. Here, the heat engine may represent a device for converting heat energy into mechanical energy, and the ORC device may represent a device that recovers waste heat having low to medium temperatures for power generation to operate a turbine. However, in the foregoing methods, when the fuel cell and the heat engine are liked with each other, or the fuel cell and the organic rankine cycle device are linked with other, it may be difficult to efficiently utilize the heat produced from the fuel cell.